1. C-31, in: R.R. Sirabian and R. Darlington (Chairs), Bioremediation and Sustainable Environmental Technologies—2013.
Second International Symposium on Bioremediation and Sustainable Environmental Technologies (Jacksonville, FL;
June 10–13, 2013). ISBN 978-0-9819730-7-4, Battelle Memorial Institute, Columbus, OH. www.battelle.org/biosymp
TCE Dechlorination Performance Assessment of a
Zero-Valent Iron Permeable Reactive Barrier at
the Former Carswell Air Force Base, Texas
Karnam Ramanand (kramanand@brwncald.com)
(Brown and Caldwell, Upper Saddle River, New Jersey, USA)
Erik McPeek (emcpeek@brwncald.com) (Brown and Caldwell, Columbus, Ohio, USA)
Adria Bodour (adria.bodour.1@us.af.mil) (AFCEE TDV, Lackland AFB, Texas, USA)
Bruce Alleman (bruce.alleman@noblis.org) (Noblis, Columbus, Ohio)
Background/Objectives. A zero-valent iron (ZVI) permeable reactive barrier (PRB) was
installed in 2002 to treat trichloroethene (TCE) in groundwater originating from closed
landfills at the former Carswell Air Force Base (AFB) (hereafter “the Site”). The PRB is
1,100 feet in length and 2 feet thick and runs from north to south with a diagonal section
that connects the northern and southern portions of the wall. Under optimal conditions, a
ZVI PRB should provide passive treatment for several decades, sufficient time to treat
substantial portions of a dissolved plume, to achieve site closure.
Approach/Activities. Increasing vinyl chloride (VC) concentrations in groundwater
downgradient of the PRB at the Site are indicative of the barrier is not performing as de-
signed. To understand the reasons for the barrier’s deterioration, an assessment was
performed, wherein the groundwater hydraulics and groundwater geochemistry were
evaluated. Groundwater geochemical trends were evaluated across 4 PRB transects, each
separated by approximately 300 feet along the barrier alignment. Each transect was com-
prised an upgradient, in-barrier, and downgradient monitoring well. Transects 1 and 2
were located in the northern portion, Transect 3 was located on the diagonal section, and
Transects 4 was located in the southern portion of the wall.
Results/Lessons Learned. At Transect 1, TCE and daughter product concentrations (cis-
and trans-1,2-dichloroethene [cDCE and tDCE] and VC) downgradient of the PRB were
low immediately after the PRB was installed in 2002 and remained low until 2007,
indicating that the PRB was effective in controlling the plume during the first 5 years.
The formation of ethene and ethane confirmed that the ZVI was achieving complete TCE
dechlorination. The pH downgradient of the barrier increased to between 8 and 10
(compared to a value of 7 to 8 in the upgradient well) and the ORP in the downgradient
well was -400 millivolts (mV) (compared to a value of of around 0 mV at the upgradient
location). These two trends as water encountered and then moved through the PRB
confirmed the reactivity of the ZVI. Data from Transect 2 showed similar trends to
Transect 1; however, the lower pH and higher ORP in the downgradient well indicated
that the groundwater flowing through the PRB near Transect 2 did not experience the
same residence time in the reactive media as at Transect 1. Data from Transects 3 and 4
showed geochemical properties at the in-barrier and downgradient locations that were
different from those observed at Transects 1 and 2. Transect 4 appeared to be least
effective in terms of plume control suggesting that certain portions of the plume at this
vicinity may be bypassing the PRB.

2. Beginning in 2008, concentrations of the hydrogenolysis breakdown products began
to increase, while the PRB was expected to primarily promote abiotic TCE dechlorination
( -elimination). The daughter products (acetylene and ethene) and geochemical (pH and
ORP) data suggested this pathway was not prevalent and, if occurring, its occurrence was
restricted to a few sections of the PRB and for a short period. There was evidence of bio-
logical activity within the PRB through the observations of TCE hydrogenolysis daughter
products within the barrier in the northern portion, as well as daughter product break-
through in the southern section. Increased methane concentrations and sulfate reduction
were indicative that the PRB had created the highly reducing conditions that favor biotic
reductive dechlorination. In summary, there was a mixed performance of the PRB, sec-
tions that were effective in treating the TCE and sections that were not effective causing
increased daughter product concentrations downgradient of the barrier.